<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>JavaScript</title>
</head>
<body>
    
</body>
<input type="button" value="弹出消息框" onclick="alert('hello world!')">
<input type="button" value="添加内容" onclick='document.write("<h1>hello world!")'>
</html>